1. Field of the Invention
The present invention relates to a photoconductor coupled liquid crystal light valve.
2. Description of the Prior Art
FIG. 5 shows the construction of a commonly used liquid crystal light valve. Referring to FIG. 5, there are disposed a photoconductive layer 11, a liquid crystal layer 2, transparent electrodes 3 provided on both sides of the photoconductive layer 11 and the liquid crystal layer 2 so as to sandwich them, a voltage applying means 4 for applying a voltage to the transparent electrodes 3, and an optical reflection layer 5 and an orientation film 6 both provided between the photoconductive layer 11 and the liquid crystal layer 2. Further, substrates 7 are disposed outside the transparent electrodes.
Next the basic principle of operation of this device is described. It is assumed for simplicity that the resistance value of the optical reflection layer is far less than those of the photoconductive layer and the liquid crystal layer. First, a voltage V.sub.0 is applied from external power supply to the photoconductive layer and the liquid crystal layer through the transparent electrodes. The voltages applied to the liquid crystal layer and the photoconductive layer are such values that V.sub.0 has been allotted in proportion to the resistance values of the two layers, where the voltage applied to the liquid crystal layer with no incident light thereon results in a value substantially less than the threshold voltage (Vsl) at which the liquid crystal layer produces the electro-optical effect. This means that the liquid crystal layer has no electro-optical effect developed in its earlier state. A schematic illustration of the charge transfer that appear in this device at this point is given in FIG. 2 by way of energy band diagram.
In the above-described state, when light is irradiated to the photoconductive layer (assuming the region where light has been applied to be Pcl, and the region where not to be Pcd), the resistance (Rp) of the photoconductive layer in Pcl reduces to a great extent, resulting in a value far smaller than the resistance of the liquid crystal, layer, so that most of Vsl greater than V.sub.0 is applied to the liquid crystal layer in Pcl. As a result, there develops an electro-optical effect to the liquid crystal layer in Pcl. In contrast to this, the liquid crystal layer in Pcd is unchange with the initial voltage applied thereto, causing no electro-optical effect to occur. Accordingly, at this point an optical pattern (optical information has been written into the liquid crystal layer. A schematic illustration of the charge transfer that appear in this device at this point is given in FIG. 3.
It is known heretofore that amorphous silicon (abbreviated as a-Si) containing hydrogen or halogen is used as the photoconductive layer (e.g. Japanese Patent Laid-Open Publications No. SHO 58-34435, SHO 58-34436, SHO 58-199327, SHO 59-81627, SHO 59-170820, etc). Otherwise, also known is use of organic photoconductive layer, amorphous selenium photoconductive layer, and the like.
However, referring to FIG. 3, optically excited carriers that have reached the a-Si layer on the side of the optical reflection layer in the light incident region must stay in the same region at least until liquid crystals complete a response in correspondence to the change in voltage. If the optically excited carriers drifted sideways so as to spread to Pcd region, the voltage applied to the liquid crystals in Pcd region would exceed Vcl, not only deteriorating the resolution but also drawing some different image from the writing image, with the resulting image disordered.
Japanese Patent Laid-Open Publication No. HEI 3-18829 has disclosed a photoconductor coupled liquid crystal light valve which uses an island-shaped metal mirror the optical reflection layer for the purpose of preventing any deterioration in the image resolution.
However, this island-shaped metal mirror not only serves as an optical reflection layer, but also functions to reduce the conductivity of the photoconductive layer in the in-plane direction, thus preventing any deterioration in resolution. Accordingly, one pixel is defined depending on the size of the island-shaped metal mirror. Moreover, making the island-shaped metal mirror involves the steps of forming a metal thin film and subsequently etching, thus adding to the process. These added processes have been a cause for increasing the cost.
In addition, there has been made no description in the prior art on the liquid crystal light valve in which special-treatment is made on the a-Si or other photoconductive layer on the side of the optical reflection layer or on its interface to prevent the arrival of carriers from the sideway drift.